Detectors which sense the presence of smoke or similar dense gases are used primarily in rooms and spaces but have a special use sensing smoke in ducts and flues conducting normally relatively clear exhause gases. It being often impractical to install the detector inside the duct, it is customary to conduct the gas to and from the detector with two pipes one upstream and one down stream of the detector. A sample of the duct gas normally flows from a higher pressure point at the upstream pipe, past the smoke detector and thence out the down stream pipe. But in an abnormal flow condition, which may be hazardous, the gas flow may stop or reverse without triggering the smoke detector and thus with no way of indicating whether normal gas flow or abnormal, possibly hazardous flow exists.
Objects of the present invention are to sense and indicate the direction of gas flow from a duct to a smoke detector.